The present invention relates to a light diffusing device for illuminating purpose to provide a surface illuminating device.
Heretofore, a light diffusing plate such as a milk glass plate or frosted glass plate been used together with a light source located behind the glass plate spaced a distance therefrom perpendicular to the illuminating surface of the glass plate in order to obtain a surface illuminating device or a two-dimensional light source.
In diffusing the light in the above described manner, however, there exists two mutually contradictory relationships between the intensity of the brightness obtained by the light diffusing glass plate and the uniformity in the distribution of the intensity of light emanated from the glass plate. The first is that the area of the light diffusing glass plate in which the brightness is held uniform becomes larger as the distance between the light source and the glass plate is made greater, whereas the intensity of the brightness is lowered. To the contrary, when the distance between the light source and the glass plate is made smaller, the area in which the intensity of the brightness is held uniform becomes smaller but the intensity of the brightness is made greater. The second is that the transmittivity of light is in general reduced to lower the brightness obtained by the diffusing glass plate when the diffusibility of the diffusing glass plate is increased, whereas the area in which the uniformity of the intensity of the brightness is maintained is made small when the transmittivity is increased.
Therefore, in order to obtain a wide area plane light source having a uniform brightness over the entire area thereof in the above described manner, it has been necessary either to provide a light source having a large power which is located at a sufficient distance from the light diffusing plate or to provide a plurality of light sources arranged in the distributed locations with respect to the diffusing plate. In either of the above described methods, however, the consumption of large electric power is required and the diffusing device is rendered to be a large size. In the former method, a severe restriction is given in the arrangement and the provision of the device, because sufficient distance must be provided between the light source and the diffusing plate, while such a device is inappropriate for use with an appliance having a thin thickness such as an illumination table, a wall hanged decoration, and an embedded advertizing device. Further, a cooling device must be provided in the device of the prior art described above.